11 La polarità è evidente fin dagli stadi inizialiThe adaxial side is towards the center of the plantPeripheralCentralAdaxialAbaxialOne way to remember which word is which – ad is like adjacent (from Latin adjacere to lie by the side of), ab is like abduct – to take away (from Latin abducere, abductum to lead away).

13 Alternata One leaf at a time, 180° apart, as in rice or other grasses.TEM of rice apexCross section of rice apexOne leaf at a time, 180° apart, as in rice or other grasses.Candela, H. et al. (2008) Plant Cell 20: ; Itoh, J.-I., et al. (2000) Plant Cell 12:

14 OppostaTwo at a time, 180° apart at each node. Sometimes pairs alternate by 90° at successive nodes.

15 VerticillataThree or more leaves at each node, as in the horsetail (Equisetum).This whorled arrangement of organs is also common in floral organs such as petals and sepals.Photos courtesy of tom donald

16 SpiralataIn most plants, such as this succulent, leaves form in a regular spiral pattern.Photos courtesy of tom donald

17 spiralataIn plants with spiral phyllotaxy, leaves form at about 137° apart.137°

26 The expected position for I3 (*) can be found by tracing the spiral.This tomato apex shows the positions of several primordia (P) and incipient primordia (I).The expected position for I3 (*) can be found by tracing the spiral.Reinhardt, D., Frenz, M., Mandel, T., and Kuhlemeier, C. (2005) Development 132: Reproduced with permission.

27 P3P2P1I1I2I1 (shown in black) was surgically isolated from the rest of the meristem, by cutting along the red line.Reinhardt, D., Frenz, M., Mandel, T., and Kuhlemeier, C. (2005) Development 132: Reproduced with permission.

28 Two days later, the apex was examined.Reinhardt, D., Frenz, M., Mandel, T., and Kuhlemeier, C. (2005) Development 132: Reproduced with permission.

30 Positions of I2 and I3; older leaves have been cut away.This scanning electron micrograph (SEM) shows that I3 formed approximately 180° away from I2. Note that the older primordia havae grown and are removed to better show the positions of I2 and I3.Reinhardt, D., Frenz, M., Mandel, T., and Kuhlemeier, C. (2005) Development 132: Reproduced with permission.

31 The older primordia control the placement of the incipient primordia.What kind of signal or information is involved?

38 Auxin transport Net flow of auxinThe subcellular localization of PIN proteins can be polar and coordinated between cells, causing directed auxin transport.In this diagram, the accumulation of PIN1 to the right of each cell causes a net flow of auxin towards the right.IAA-pH 7pH 5IAAHNet flow of auxin

41 Conclusion - Auxin transport and a local auxin maximum contribute to organ initiation.This conclusion is supported by imaging PIN1 distribution in living plants.IAA-pH 7pH 5IAAH

42 Visualizing PIN1 localizationEmitted lightPIN1GFPGFPExcitatory lightGreen fluorescent protein (GFP) emits green light when excited by blue light.A protein’s position within a cell can be determined by making a fusion protein of it with GFP, and then looking for GFP fluorescence.

52 SummaryOrgan initiation at the shoot apical meristem is determined by auxin distribution and PIN1An auxin maximum is necessary and sufficient to specify the site of primordium formationPrimordia affect auxin distribution and so placement of incipient primordiaAuxin has been proposed to act as a morphogen – a generator of form

72 SummaryA leaf acquires identity by turning OFF meristem genes and turning ON leaf genesKNOX-1, ARP and boundary genes encode transcriptional regulators that control expression of other genesPrecise control of cell fates involves tight control of transcription by developmentally regulated activators and repressors

75 LeafPeripheralCentralLeaves have an inherent polarity because one side is more central and one more peripheral.

76 The central side is adaxial, and peripheral is abaxial.LeafPeripheralCentralAdaxialAbaxialThe central side is adaxial, and peripheral is abaxial.How does a leaf “know” which side is central?

77 The Sussex signalIn the 1950s, Ian Sussex showed that a signal from the meristem is required for proper leaf polarity.Reprinted, with permission, from the Annual Review of Plant Physiology and Plant Molecular Biology, Volume 49 (c) 1998 by Annual Reviews

78 Incipient primordia were surgically isolated from the rest of the meristem by a small incisionReinhardt, D., Frenz, M., Mandel, T., and Kuhlemeier, C. (2005) Development 132: Reproduced with permission.

79 The isolated primordium lost polarity (it became entirely abaxialized) and became radially-symmetrical.P3P2P1I1I2I3P3P2P1I1I2I3

80 A more recent experiment showed that laser ablation of only the epidermal cell layer is sufficient for the primordium to lose its adaxial polarity.Reinhardt, D., Frenz, M., Mandel, T., and Kuhlemeier, C. (2005) Development 132: Reproduced with permission.

81 A signal from the meristem moves through the epidermis into the incipient primordium.The signal conveys the adaxial positional information.The nature of the signal is not known.

104 SUMMARY Leaves are initiated from cells in the shoot apical meristemAuxin gradients are important in leaf primordium initiation and positioningLeaf identity is determined by a change in expression of transcription-factor encoding genesLeaf polarity requires an unknown signal from the meristem and the domain-specific expression of adaxial- and abaxial- specific transcription factors